Devices for measuring the water vapor content of the atmosphere by means of spectroscopic methods have been known for a long time. The absorption spectrum of water vapor molecules extends across a very broad range of the spectrum. Devices are in use which operate at 1,215.6 .ANG., i.e. on the wave length of the so-called Lyman .alpha. line (these devices will be called L.alpha. devices hereinbelow). Furthermore, devices are known in which the absorption of water vapor in the infrared range of the spectrum is used for determining moisture (these devices are designated as infrared devices hereinbelow). Additionally, devices are known which operate in the microwave range; however, these devices will not be further examined herein.
All spectroscopic determinations of concentration proceed from the so-called Beer's law which for parallel beam paths reads: EQU I.sub..lambda. =I.sub..lambda..exp(-K.sub..lambda. X) (1)
where, with respect to a set wavelength .lambda., the value I.sub..lambda. indicates the beamed intensity, the value I.sub..lambda. indicates the intensity of the beam after passing the absorption distance having a length X, and the value K.sub..lambda. indicates the absorption coefficient.
In the infrared range of the spectrum, the absorption spectrum of water vapor consists of a plurality of single lines, so-called bands, which no longer can be distinguished by customary infrared hygrometers. In place of the various absorption coefficients K.sub..lambda. which apply for the individual lines, an empirical formula applies instead of equation (1), which is mathematically awkward to handle and therefore approximation equations are often used.
Therefore the association of moisture values to the measured intensities is performed by corresponding calibration methods, in which the properties of the devices, such as filter transparency, amplification, detector sensitivity, etc., as well as the pressure dependence, must also be considered.
The spectroscopically operating hygrometers can be assigned to various groups. The most simple in principle are the so-called single beam devices (single beam hygrometers), such as described, e.g. by Hyson, Hicks in J.Appl.Met. 14, 1975, pp. 301 to 307. The customary L.alpha. devices also operate as single beam devices. In connection with these single beam devices constant operating conditions must be strictly maintained in order to obtain truly comparable values. In spite of this, frequent test calibrations are necessary to measure changes extending over long periods of time.
It is possible to achieve better accuracy with so-called double beam devices (double beam hygrometers), such as described by Bogomovola et al in Izv.Atm. and Oceanic Phys., vol. 10, 1974, pp. 933 to 942. Comparative measurements are taken with these double beam devices, in particular in a wave range in which the water vapor is absorbed very little or not at all. By developing a quotient it is possible in this way to compensate to a large degree for slow changes in the device.
Often a chopper is introduced into the beam path in spectroscopically operating hygrometers. By the use of this, alternating voltage is obtained for use as signal voltage. This facilitates amplification and filtering. In double beam devices either two separate detectors can be used which, however, must be operated under exactly the same constant conditions, or it is also possible to operate with only one detector by, e.g., alternatingly introducing two filters into the beam path or by modulating (chopping) the two beams with different frequencies.
For physically technical reasons it is not possible to operate with two beams of different wavelength in L.alpha. devices. Therefore it has been suggested, e.g. by Buck in Bull.Am.Met.Soc, vol. 57, 1976, pp. 1113 to 1118, to change the absorption distance X. Because L.alpha. devices in general have a very short absorption distance, small changes of the value X (see Equation (1)) are very effective. However, a very high degree of accuracy in adjustment is required in this case. A further device for the measurement of water vapor density and of a saturation ratio is known from U.S. Pat. No. 4,394,575.
In spite of the development of a quotient, certain influences which lead to errors in measurements cannot be eliminated in hygrometers operating with two wavelengths. This is the case, for example, if the distribution of spectral intensity of a beam source or the distribution of spectral sensitivity of a detector changes in dependence on the wavelength. For these reasons corrections can only be made by means of calibration tests made at certain time intervals.
However, it is possible that an adsorption layer or even a coating forms on the surfaces of windows, lenses or mirrors, which, because they have an absorption depending on the wavelength, result in false measurement values. Latest measurements under special laboratory conditions have indicated that at dew point temperatures around -40.degree. C. it is possible to get a consistent error of almost 10.degree. C. in the determination of the dew point, which error is due to adsorption. It has been suggested to heat the windows, lenses or mirrors to avoid coatings, but this also does not assure that an adsorption layer is reliably avoided.